A nuclear instrumentation system, also known as the RPN system, employs a series of detectors distributed outside the reactor pressure vessel for measuring a reactor power, a power variation rate and a power radial and axial distribution and so on. At present, the RPN system is known to contain two source range channel detectors, two intermediate range channel detectors, and four power range channel detectors. The intermediate range channel detector consists of several compensated ionization chambers, and the power range channel detector consists of six segments of long ionization chambers. As the long ionization chambers and the compensated ionization chambers are employed, the intermediate range channel detector and the power range channel detector have a poor performance in Gamma radiation resistance property, anti-noise property and anti-electromagnetic interference property, thus having poor stability and reliability, so can not satisfy the surveilling requests of the nuclear power plant after accidents.
In additional, the source range channel and intermediate range channel have two channels which possessing a lower redundancy. Consequently, when an accident occurs, the source range channel and intermediate range channel have signals of low reliability. Further, in the traditional RPN system, the distribution in the radial direction and the axial direction is as follows. The standby channel is located at the radial positions of 0° and 180° of the reactor pressure vessel in the distribution along the radial direction, the source range channel detector and intermediate range channel detector are located at the radial positions of 90° and 270° of the reactor pressure vessel in the distribution along the radial direction, the power range channel detectors are located at the radial positions of 45°, 135°, 225° and 315° of the reactor pressure vessel in the distribution along the radial direction, respectively. In the distribution along the axial direction, the source range channel detector and intermediate range channel detector are centred in a core plane at ¼ or ½ height of the core, the power range channel detector is centred in a core plane at ½ height of the core. When locating, the cylinder bracket of the detector can not be directly installed in the measurement position due to the support ring of the pressure vessel. Referring FIG. 1, the RPN system locates the detectors via a push-pull trolley. As shown in FIG. 1, reference number 1 represents the vessel head of the pressure vessel, reference number 2 represents the cover, reference number 3 represents the cable connecting plate, reference number 4 represents the detector, reference number 5 represents the core, reference number 6 represents the measuring position, reference number 7 represents the position when the detector is pulled out, reference number 8 represents the moveable locating device. The source range channel detector and the intermediate range channel detector are installed in the same sleeve, the power range channel detector is installed in another sleeve. These sleeves enter the locating trolley via lifting channels in different angles and are fixed therein (referring the detection tube shown by dotted line in FIG. 1). Then, these sleeves are transferred to the work position along the predetermined route via the locating trolley, then the cable is connected with the cable connecting plate via an upper hole. The detectors can be lifted in and out via the upper hole for maintenance and installation. However, not only is such installation and location manner cumbersome, but also it requires large space for installation and maintenance and is inconvenient for operation and maintenance. Even in the test maintenance during refueling outage, the work staff also suffers much radioactive irradiation, and they would suffer even more when inspecting or replacing the detectors. In addition, the cable connecting plate is very close to the reactor core, so its thermal expansion and contraction is likely to result in connection loose, thus mutating or losing measuring signal of the detector.